The present invention relates to turbine flowmeters.
A turbine flowmeter is described here having an impeller wheel mounted, so that it can rotate, between an upstream hub situated on an upstream side and a downstream hub situated on a downstream side in a tubular element. The hubs are, in each case, mounted in the tubular element in a fixed relationship to the tubular element by ribs. Means for regulating to compensate measuring errors resulting from manufacturing tolerances are provided at least on the upstream hub.
Due to manufacturing tolerances, standard turbine flowmeters exhibit slight measuring errors which must be regulated, i.e. minimized by suitable measures.
According to the state of the art it is known to form one of the ribs securing the hub situated upstream of the impeller wheel as a regulating vane, whereby for this purpose the rib is mounted so that it can rotate about a radial axis. In its off position this regulating vane is oriented in the flow direction of the fluid to be measured. If the regulating vane is rotated into the flow, the flow experiences a change of direction. The flow influenced in this way is also received by the impeller wheel, i.e. its speed of rotation increases or decreases depending on the direction in which the regulating vane is swung out. Indicator deviations brought about as a result of the manufacturing process can be compensated with the help of this control device comprising the regulating vane.
In the case of a reverse flow, the regulating vane described is in a position downstream of the impeller wheel and, consequently, does not have any effect on the rotational speed of the impeller wheel. To be able to regulate when the flow is in the reverse direction as well, a second, additional regulating vane is incorporated in a similar way on the other side of the impeller wheel. Therefore, measuring errors of the turbine flowmeter can be compensated in both flow directions with this type of control device.
With this known arrangement, when the regulating vane is swung out, besides the desired change of flow direction, an undesired asymmetry in the flow against the impeller wheel is produced. This results in additional forces in a certain direction transverse to the flow on the impeller wheel and on its bearings which are then subject to more rapid, one-sided wear in this direction. Apart from that, the amount by which the turbine flowmeter can be regulated with this arrangement depends on the flow velocity of the fluid. In particular, the regulating effect steadily deteriorates as the flow rate decreases until the effect disappears completely.